Network Working Group H. S. Thompson
Internet-Draft University of Edinburgh
Obsoletes: 3023 (if approved) C. Lilley
Updates: 6839 (if approved) W3C
Intended status: Standards Track November 04, 2013
Expires: May 08, 2014
XML Media Typesdraft-ietf-appsawg-xml-mediatypes-04
Abstract
This specification standardizes three media types -- application/xml,
application/xml-external-parsed-entity, and application/xml-dtd --
for use in exchanging network entities that are related to the
Extensible Markup Language (XML) while defining text/xml and text/
xml-external-parsed-entity as aliases for the respective application/
types. This specification also standardizes the '+xml' suffix for
naming media types outside of these five types when those media types
represent XML MIME entities.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 08, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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Internet-Draft XML Media Types November 20131. Introduction
The World Wide Web Consortium has issued the Extensible Markup
Language (XML) 1.0 [XML] and Extensible Markup Language (XML) 1.1
[XML1.1] specifications. To enable the exchange of XML network
entities, this specification standardizes three media types --
application/xml, application/xml-external-parsed-entity, and
application/xml-dtd and two aliases -- text/xml and text/xml-
external-parsed-entity, as well as a naming convention for
identifying XML-based MIME media types (using '+xml').
XML has been used as a foundation for other media types, including
types in every branch of the IETF media types tree. To facilitate
the processing of such types, and in line with the recognition in
[RFC6838] of structured syntax name suffixes, a suffix of '+xml' is
described in Section 8. This will allow generic XML-based tools --
browsers, editors, search engines, and other processors -- to work
with all XML-based media types.
This specification replaces [RFC3023]. Major differences are in the
areas of alignment of charset handling for text/xml and text/xml-
external-parsed-entity with application/xml, the addition of XPointer
and XML Base as fragment identifiers and base URIs, respectively,
integration of the XPointer Registry and updating of many references.
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
specification are to be interpreted as described in [RFC2119].
As defined in [RFC2781] (informative), the three character sets
"utf-16", "utf-16le", and "utf-16be" are used to label UTF-16 text.
In this specification, "the UTF-16 family" refers to those three
character sets. By contrast, the phrases "utf-16" or UTF-16 in this
specification refer specifically to the single charset "utf-16".
As sometimes happens between two communities, both MIME and XML have
defined the term entity, with different meanings. Section 2.4 of
[RFC2045] says:
"The term 'entity' refers specifically to the MIME-defined header
fields and contents of either a message or one of the parts in the
body of a multipart entity."
Section 4 of [XML] says:
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"An XML document may consist of one or many storage units. These
are called entities; they all have content and are all (except for
the document entity and the external DTD subset) identified by
entity name".
In this specification, "XML MIME entity" is defined as the latter (an
XML entity) encapsulated in the former (a MIME entity).
Furthermore, XML provides for the naming and referencing of entities
for purposes of inclusion and/or substitution. In this specification
"XML-entity declaration/reference/..." is used to avoid confusion
when referring to such cases.
3. XML Media Types
Registration information for media types for use with XML MIME
entities is described in the sections below. Within the XML
specification, such entities can be classified into four types. In
the XML terminology, they are called "document entities", "external
DTD subsets", "external parsed entities", and "external parameter
entities". Appropriate usage for the types registered below is as
follows:
document entities The media types application/xml or text/xml MAY be
used.
external DTD subsets The media type application/xml-dtd SHOULD be
used. The media types application/xml and text/xml MUST NOT be
used.
external parsed entities The media types application/xml-external-
parsed-entity or text/xml-external-parsed-entity SHOULD be used.
The media types application/xml and text/xml MUST NOT be used
unless the parsed entities are also well-formed "document
entities" and are referenced as such.
external parameter entities The media type application/xml-dtd
SHOULD be used. The media types application/xml and text/xml MUST
NOT be used.
Note that [RFC3023] (which this specification obsoletes)
recommended the use of text/xml and text/xml-external-parsed-
entity for document entities and external parsed entities,
respectively, but described charset handling which differed from
common implementation practice. These media types are still
commonly used, and this specification aligns the charset handling
with industry practice.
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Note that [RFC2376] (which is obsolete) allowed application/xml
and text/xml to be used for any of the four types, although in
practice it is likely to have been rare.
Neither external DTD subsets nor external parameter entities parse as
XML documents, and while some XML document entities may be used as
external parsed entities and vice versa, there are many cases where
the two are not interchangeable. XML also has unparsed entities,
internal parsed entities, and internal parameter entities, but they
are not XML MIME entities.
Compared to [RFC2376] or [RFC3023], this specification alters the
charset handling of text/xml and text/xml-external-parsed-entity,
treating them no differently from the respective application/ types,
however application/xml and application/xml-external-parsed-entity
are still RECOMMENDED, to avoid possible confusion based on the
earlier distinction. The former confusion around the question of
default character sets for the text/xml... types has been resolved by
[HTTPbis] changing [RFC2616] by removing the ISO-8859-1 default and
not defining any default at all, as well as [RFC6657] updating
[RFC2046] to remove the US-ASCII default. See Section 3.6 for the
now-unified approach to the charset parameter which results.
XML provides a general framework for defining sequences of structured
data. It is often appropriate to define new media types that use XML
but define a specific application of XML, due to domain-specific
display, editing, security considerations or runtime information.
Furthermore, such media types may allow only UTF-8 and/or UTF-16 and
prohibit other character sets. This specification does not prohibit
such media types and in fact expects them to proliferate. However,
developers of such media types are RECOMMENDED to use this
specification as a basis for their registration. See Section 8 for
more detailed recommendations on using the '+xml' suffix for
registration of such media types.
An XML document labeled as application/xml or text/xml, or with a
'+xml' media type, might contain namespace declarations, stylesheet-
linking processing instructions (PIs), schema information, or other
declarations that might be used to suggest how the document is to be
processed. For example, a document might have the XHTML namespace
and a reference to a CSS stylesheet. Such a document might be
handled by applications that would use this information to dispatch
the document for appropriate processing.
3.1. Application/xml Registration
Type name: application
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Subtype name: xml
Required parameters: none
Optional parameters: charset
See Section 3.6.
Encoding considerations: Depending on the charset encoding used, XML
MIME entities can consist of 7bit, 8bit or binary data [RFC6838].
For 7-bit transports, 7bit data, for example data with charset
encoding US-ASCII, does not require content-transfer-encoding, but
8bit or binary data, for example data with charset encoding UTF-8
or UTF-16, MUST be content-transfer-encoded in quoted-printable or
base64. For 8-bit clean transport (e.g. 8BITMIME [RFC6152],
ESMTP or NNTP [RFC3977]), 7bit or 8bit data, for example data with
charset encoding UTF-8 or US-ASCII, does not require content-
transfer-encoding, but binary data, for example data with a
charset encoding from the UTF-16 family, MUST be content-transfer-
encoded in base64. For binary clean transports (e.g. HTTP
[RFC2616]), no content-transfer-encoding is necessary (or even
possible, in the case of HTTP) for 7bit, 8bit or binary data.
Security considerations: See Section 11.
Interoperability considerations: XML has proven to be interoperable
across both generic and task-specific applications and for import
and export from multiple XML authoring and editing tools.
Validating processors provide maximum interoperability. Although
non-validating processors may be more efficient, they are not
required to handle all features of XML. For further information,
see sub-section 2.9 "Standalone Document Declaration" and section5 "Conformance" of [XML] .
Published specification: Extensible Markup Language (XML) 1.0 (Fifth
Edition) [XML] or subsequent editions or versions thereof.
Applications that use this media type: XML is device-, platform-,
and vendor-neutral and is supported by a wide range of generic XML
tools (editors, parsers, Web agents, ...), generic and task-
specific applications.
Additional information:
Magic number(s): None.
Although no byte sequences can be counted on to always be
present, XML MIME entities in ASCII-compatible character sets
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documents cannot be reliably used as external parsed entities
because external parsed entities are prohibited from having
standalone document declarations or DTDs. Identifying XML
external parsed entities with their own content type should
enhance interoperability of both XML documents and XML external
parsed entities.
Published specification: Same as application/xml as described in
Section 3.1.
Applications which use this media type: Same as application/xml as
described in Section 3.1.
Additional information:
Magic number(s): Same as application/xml as described in
Section 3.1.
File extension(s): .xml or .ent
Macintosh File Type Code(s): "TEXT"
Base URI: See Section 6
Person and email address for further information: See Authors'
Addresses section.
Intended usage: COMMON
Author: See Authors' Addresses section.
Change controller: The XML specification is a work product of the
World Wide Web Consortium's XML Core Working Group
3.4. Text/xml-external-parsed-entity Registration
text/xml-external-parsed-entity is an alias for application/xml-
external-parsed-entity, as defined in Section 3.3 above.
3.5. Application/xml-dtd Registration
Type name: application
Subtype name: xml-dtd
Required parameters: none
Optional parameters: charset
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See Section 3.6.
Encoding considerations: Same as Section 3.1.
Security considerations: See Section 11.
Interoperability considerations: XML DTDs have proven to be
interoperable by DTD authoring tools and XML validators, among
others.
Published specification: Same as application/xml as described in
Section 3.1.
Applications which use this media type: DTD authoring tools handle
external DTD subsets as well as external parameter entities. XML
validators may also access external DTD subsets and external
parameter entities.
Additional information:
Magic number(s): Same as application/xml as described in
Section 3.1.
File extension(s): .dtd or .mod
Macintosh File Type Code(s): "TEXT"
Person and email address for further information: See Authors'
Addresses section.
Intended usage: COMMON
Author: See Authors' Addresses section.
Change controller: The XML specification is a work product of the
World Wide Web Consortium's XML Working Core Group
3.6. Charset considerations
As many as three distinct sources of information about character
encoding may be present for an XML MIME entity: a charset parameter,
a Byte Order Mark (BOM -- see Section 4 below) and an XML encoding
declaration (see Section 4.3.3 of [XML]). Ensuring consistency among
these sources requires coordination between entity authors and MIME
agents (that is, processes which package, transfer, deliver and/or
receive MIME entities). Some MIME agents will be what we will call
"XML-aware", that is, capable of processing XML MIME entities and
detecting the XML encoding declaration (or its absence). Others will
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not be XML-aware, and thus cannot know anything about the XML
encoding declaration. Some MIME agents, such as proxies and
transcoders, both consume and produce MIME entities.
XML-aware MIME producers SHOULD supply a charset parameter and/or an
appropriate BOM with non-UTF-8-encoded XML MIME entities which lack
an encoding declaration, and SHOULD remove or correct an encoding
declaration which is known to be incorrect (for example, as a result
of transcoding).
XML-unaware MIME producers MUST NOT supply a charset parameter with
an XML MIME entity unless the entity's character encoding is reliably
known.
XML MIME producers are RECOMMENDED to provide means for XML MIME
entity authors to control the supply of charset parameters for their
entities, for example by enabling user-level configuration of
filename-to-Content-Type-header mappings on a file-by-file or suffix
basis.
For XML MIME consumers, the question of priority arises in cases when
the available character encoding information is not consistent.
Again, we must distinguish betweeen XML-aware and XML-unaware
processors.
When a charset parameter is specified for an XML MIME entity, then
regardless of whether or not the entity contains in-band encoding
information, that is, either a BOM (Section 4) or an XML encoding
declaration or both, or none, the normative component of the [XML]
specification leaves the question open as to how to determine the
encoding with which to attempt to process the entity. In particular,
in the case where there is in-band information and it conflicts with
the charset parameter, the [XML] specification does not specify which
should be taken to be authoritative. In its (non-normative)
Appendix F it defers to this specification:
[T]he preferred method of handling conflict should be specified as
part of the higher-level protocol used to deliver XML. In
particular, please refer to [IETF RFC 3023] or its successor
Accordingly, to conform with deployed processors and content and to
avoid conflicting with this or other normative specifications, this
specification sets the priority as follows:
All consumers SHOULD treat a BOM (Section 4) as authoritative if it
is present in an XML MIME entity. In the absence of a BOM
(Section 4), all consumers SHOULD treat the charset parameter as
authoritative if it is present. For XML-aware consumers, note that
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Internet-Draft XML Media Types November 2013Section 4.3.3 of the [XML] specification does _not_ make it an error
for the charset parameter and the XML encoding declaration (or the
UTF-8 default in the absence of encoding declaration and BOM) to be
inconsistent, although such processors might choose to issue a
warning in this case.
When MIME producers conform to the requirements on them stated above,
such inconsistencies will not arise---this statement of priorities
only has practical impact in the case of non-conforming XML MIME
entities.
If an XML MIME entity is received where the charset parameter is
omitted, no information is being provided about the charset by the
MIME Content-Type header. XML-aware processors MUST follow the
requirements in section 4.3.3 of [XML] that directly address this
case. XML-unaware MIME processors SHOULD NOT assume a default
charset in this case.
4. The Byte Order Mark (BOM) and Charset Conversions
Section 4.3.3 of [XML] specifies that XML MIME entities in the
charset "utf-16" MUST begin with a byte order mark (BOM), which is a
hexadecimal octet sequence 0xFE 0xFF (or 0xFF 0xFE, depending on
endian). The XML Recommendation further states that the BOM is an
encoding signature, and is not part of either the markup or the
character data of the XML document.
Due to the presence of the BOM, applications that convert XML from
"utf-16" to an encoding other than "utf-8" MUST strip the BOM before
conversion. Similarly, when converting from another encoding into
"utf-16", the BOM MUST be added after conversion is complete unless
the original encoding was "utf-8" and a BOM was already present, in
which case it will have been transcoded into a "utf-16" BOM already.
Section 4.3.3 of [XML] also allows for XML MIME entities in the
charset "utf-8" to begin with a byte order mark (BOM), which is a
hexadecimal octet sequence 0xEF 0xBB 0xBF, also defined to be an
encoding signature, and not part of either the markup or the
character data of the XML document.
Applications that convert XML from "utf-8" to an encoding other than
"utf-16" MUST strip the BOM, if present, before conversion.
Applications which convert XML into "utf-8" SHOULD add a BOM after
conversion is complete.
In addition to the charset "utf-16", [RFC2781] introduces "utf-16le"
(little endian) and "utf-16be" (big endian) as well. The BOM is
prohibited for these character sets. When an XML MIME entity is
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encoded in "utf-16le" or "utf-16be", it MUST NOT begin with the BOM
but SHOULD contain an in-band XML encoding declaration. Conversion
from "utf-16"or "utf-8" to "utf-16be" or "utf-16le" and conversion in
the other direction MUST strip or add the appropriate BOM,
respectively.
5. Fragment Identifiers
Uniform Resource Identifiers (URIs) can contain fragment identifiers
(see Section 3.5 of [RFC3986]).Specifying the syntax and semantics of
fragment identifiers is devolved by [RFC3986] to the appropriate
media type registration.
The syntax and semantics of fragment identifiers for the XML media
types defined in this specification are based on the
[XPointerFramework] W3C Recommendation. It allows simple names, and
more complex constructions based on named schemes. When the syntax
of a fragment identifier part of any URI or IRI with a retrieved
media type governed by this specification conforms to the syntax
specified in [XPointerFramework], conforming applications MUST
interpret such fragment identifiers as designating whatever is
specified by the [XPointerFramework] together with any other
specifications governing the XPointer schemes used in those
identifiers which the applications support. Conforming applications
MUST support the 'element' scheme as defined in [XPointerElement],
but need not support other schemes.
If an XPointer error is reported in the attempt to process the part,
this specification does not define an interpretation for the part.
A registry of XPointer schemes [XPtrReg] is maintained at the W3C.
Document authors SHOULD NOT use unregistered schemes. Scheme authors
SHOULD register their schemes ([XPtrRegPolicy] describes requirements
and procedures for doing so).
See Section 8.1 for additional requirements which apply when an XML-
based media type follows the naming convention '+xml'.
If [XPointerFramework] and [XPointerElement] are inappropriate for
some XML-based media type, it SHOULD NOT follow the naming convention
'+xml'.
When a URI has a fragment identifier, it is encoded by a limited
subset of the repertoire of US-ASCII [ASCII] characters, as defined
in [RFC3986].
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Internet-Draft XML Media Types November 2013Section 5.1 of [RFC3986] specifies that the semantics of a relative
URI reference embedded in a MIME entity is dependent on the base URI.
The base URI is established by (1) the base URI embedded in content,
(2) the base URI from the encapsulating entity, (3) the base URI from
the Retrieval URI, or (4) the default base URI, in order of
precedence. [RFC3986] further specifies that the mechanism for
embedding the base URI is dependent on the media type.
This specification accordingly provides the following media type
dependent mechanism for embedding the base URI in a MIME entity of
type application/xml, text/xml, application/xml-external-parsed-
entity or text/xml-external-parsed-entity: An XML MIME entity MAY use
the xml:base attribute, as described in detail in [XMLBase], to
establish a base URI for that entity.
Note that the base URI itself might be embedded in a different MIME
entity, since the default value for the xml:base attribute can be
specified in an external DTD subset or external parameter entity.
Since conforming XML processors need not always read and process
external entities, the effect of such an external default is
uncertain and therefore its use is NOT RECOMMENDED.
7. XML Versions
application/xml, application/xml-external-parsed-entity, and
application/xml-dtd, text/xml and text/xml-external-parsed-entity are
to be used with [XML]. In all examples herein where version="1.0" is
shown, it is understood that version="1.1" might also appear,
providing the content does indeed conform to [XML1.1].
The normative requirement of this specification upon XML documents
and processors is to follow the requirements of [XML], section 4.3.3.
Except for minor clarifications, that section is substantially
identical from the first edition to the current (5th) edition of XML
1.0, and for XML 1.1 1st or 2nd edition [XML1.1]. Therefore,
references herein to [XML] may be interpreted as referencing any
existing version or edition of XML, or any subsequent edition or
version which makes no incompatible changes to that section.
Specifications and recommendations based on or referring to this RFC
SHOULD indicate any limitations on the particular versions or
editions of XML to be used.
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Internet-Draft XML Media Types November 20138. A Naming Convention for XML-Based Media Types
This section supersedes the earlier registration of the '+xml' suffix
[RFC6839].
This specification recommends the use of a naming convention (a
suffix of '+xml') for identifying XML-based media types, in line with
the recognition in [RFC6838] of structured syntax name suffixes.
This allows the use of generic XML processors and technologies on a
wide variety of different XML document types at a minimum cost, using
existing frameworks for media type registration.
When a new media type is introduced for an XML-based format, the name
of the media type SHOULD end with '+xml' unless generic XML
processing is in some way inappropriate for documents of the new
type. This convention will allow applications that can process XML
generically to detect that the MIME entity is supposed to be an XML
document, verify this assumption by invoking some XML processor, and
then process the XML document accordingly. Applications may match
for types that represent XML MIME entities by comparing the subtype
to the pattern '*/*+xml'. (However note that 4 of the 5 media types
defined in this specification -- text/xml, application/xml, text/xml-
external-parsed-entity, and application/xml-external-parsed-entity --
also represent XML MIME entities while not conforming to the '*/
*+xml' pattern.)
NOTE: Section 5.3.2HTTPbis [HTTPbis] does not support Accept
headers of the form "Accept: */*+xml" and so this header MUST NOT
be used in this way.
Media types following the naming convention '+xml' SHOULD introduce
the charset parameter for consistency, since XML-generic processing
applies the same program for any such media type. However, there are
some cases that the charset parameter need not be introduced. For
example:
When an XML-based media type is restricted to UTF-8, it is not
necessary to introduce the charset parameter. "UTF-8 only" is a
generic principle and UTF-8 is the default of XML.
When an XML-based media type is restricted to UTF-8 and UTF-16, it
might not be unreasonable to omit the charset parameter. Neither
UTF-8 nor UTF-16 require in-band XML encoding declarations.
XML generic processing is not always appropriate for XML-based media
types. For example, authors of some such media types may wish that
the types remain entirely opaque except to applications that are
specifically designed to deal with that media type. By NOT following
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the naming convention '+xml', such media types can avoid XML-generic
processing. Since generic processing will be useful in many cases,
however -- including in some situations that are difficult to predict
ahead of time -- the '+xml' convention is to be preferred unless
there is some particularly compelling reason not to.
The registration process for specific '+xml' media types is described
in [RFC6838]. The registrar for the IETF tree will encourage new
XML-based media type registrations in the IETF tree to follow this
guideline. Registrars for other trees SHOULD follow this convention
in order to ensure maximum interoperability of their XML-based
documents. Similarly, media subtypes that do not represent XML MIME
entities MUST NOT be allowed to register with a '+xml' suffix.
8.1. Referencing
Registrations for new XML-based media types under top-level types
SHOULD, in specifying the charset parameter and encoding
considerations, define them as: "Same as [charset parameter /
encoding considerations] of application/xml as specified in RFC
XXXX."
Enabling the charset parameter is RECOMMENDED, since this information
can be used by XML processors to determine authoritatively the
charset of the XML MIME entity in the absence of a BOM. If there are
some reasons not to follow this advice, they SHOULD be included as
part of the registration. As shown above, two such reasons are
"UTF-8 only" or "UTF-8 or UTF-16 only".
These registrations SHOULD specify that the XML-based media type
being registered has all of the security considerations described in
RFC XXXX plus any additional considerations specific to that media
type.
These registrations SHOULD also make reference to RFC XXXX in
specifying magic numbers, base URIs, and use of the BOM.
These registrations MAY reference the application/xml registration in
RFC XXXX in specifying interoperability considerations, if these
considerations are not overridden by issues specific to that media
type.
8.2. +xml Structured Syntax Suffix Registration
Name: Extensible Markup Language (XML)
+suffix: +xml
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Reference: This specification
Encoding considerations: Same as Section 3.1.
Fragment identifier considerations: Registrations which use this
'+xml' convention MUST also make reference to RFC XXXX,
specifically Section 5, in specifying fragment identifier syntax
and semantics, and they MAY restrict the syntax to a specified
subset of schemes, except that they MUST NOT disallow barenames or
'element' scheme pointers. They MAY further require support for
other registered schemes. They also MAY add additional syntax
(which MUST NOT overlap with [XPointerFramework] syntax) together
with associated semantics, and MAY add additional semantics for
barename XPointers which, as provided for in Section 5, will only
apply when this specification does not define an interpretation.
In practice these constraints imply that for a fragment
identifier addressed to an instance of a specific "xxx/yyy+xml"
type, there are three cases:
For fragment identifiers matching the syntax defined in
[XPointerFramework], where the fragment identifier resolves
per the rules specified there, then process as specified
there;
For fragment identifiers matching the syntax defined in
[XPointerFramework], where the fragment identifier does
_not_ resolve per the rules specified there, then process as
specified in "xxx/yyy+xml";
For fragment identifiers _not_ matching the syntax defined
in [XPointerFramework], then process as specified in "xxx/
yyy+xml".A fragment identifier of the form
"xywh=160,120,320,240", as defined in [MediaFrags], which
might be used in a URI for an XML-encoded image, would fall
in this category.
Interoperability considerations: Same as Section 3.1. See above,
and also Section 3.6, for guidelines on the use of the 'charset'
parameter.
Security considerations: See Section 11.
Contact: See Authors' Addresses section.
Author: See Authors' Addresses section.
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Change controller: The XML specification is a work product of the
World Wide Web Consortium's XML WorkingCore Group.
9. Examples
The examples below give the charset portion, if any, of the value of
the MIME Content-type header and the XML declaration or Text
declaration (which includes the encoding declaration) inside the XML
MIME entity. For UTF-16 examples, the Byte Order Mark character
appropriately UTF-16-encoded is denoted as "{BOM}", and the XML or
Text declaration is assumed to come at the beginning of the XML MIME
entity, immediately following the encoded BOM. Note that other MIME
headers may be present, and the XML MIME entity may contain other
data in addition to the XML declaration; the examples focus on the
Content-type header and the encoding declaration for clarity.
All the examples below apply to all five media types declared above
in Section 3, as well as to any media types declared using the '+xml'
convention (with the exception of the examples involving the charset
parameter for any such media types which do not enable its use). See
the XML MIME entities table (Section 3, Paragraph 2) for discussion
of which types are appropriate for which varieties of XML MIME
entities.
This section is non-normative. In particular, note that all
[RFC2119] language herein reproduces or summarizes the consequences
of normative statements already made above, and has no independent
normative force, and accordingly does not appear in uppercase.
9.1. UTF-8 Charset
Content-type charset: charset="utf-8"
<?xml version="1.0" encoding="utf-8"?>
This is the recommended encoding for use with all the media types
defined in this specification. Since the charset parameter is
provided and there is no BOM, both MIME and XML processors must treat
the enclosed entity as UTF-8 encoded.
If sent using a 7-bit transport (e.g. SMTP [RFC5321]), the XML MIME
entity must use a content-transfer-encoding of either quoted-
printable or base64. For an 8-bit clean transport (e.g. 8BITMIME,
ESMTP or NNTP), or a binary clean transport (e.g. HTTP), no content-
transfer-encoding is necessary (or even possible, in the case of
HTTP).
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Content-type charset: charset="utf-16"
{BOM}<?xml version="1.0" encoding="utf-16"?>
or
{BOM}<?xml version="1.0"?>
For application/... cases, if sent using a 7-bit transport (e.g.
SMTP) or an 8-bit clean transport (e.g. 8BITMIME, ESMTP or NNTP),
the XML MIME entity must be encoded in quoted-printable or base64;
for a binary clean transport (e.g. HTTP), no content-transfer-
encoding is necessary (or even possible, in the case of HTTP).
As described in [RFC2781], the UTF-16 family must not be used with
media types under the top-level type "text" except over HTTP or HTTPS
(see section 19.4.2 of [RFC2616] for details). Hence this example is
only possible in text/... cases when the XML MIME entity is
transmitted via HTTP or HTTPS, which use a MIME-like mechanism and
are binary-clean protocols, hence do not perform CR and LF
transformations and allow NUL octets. Since HTTP is binary clean, no
content-transfer-encoding is necessary (or even possible).
9.3. Omitted Charset and 8-bit MIME entity
Content-type charset: [none]
<?xml version="1.0" encoding="iso-8859-1"?>
Since the charset parameter is not provided in the Content-Type
header and there is no BOM, XML processors must treat the
"iso-8859-1" encoding as authoritative. XML-unaware MIME processors
should make no assumptions about the charset of the XML MIME entity.
9.4. Omitted Charset and 16-bit MIME entity
Content-type charset: [none]
{BOM}<?xml version="1.0" encoding="utf-16"?>
or
{BOM}<?xml version="1.0"?>
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This example shows a 16-bit MIME entity with no charset parameter.
However since there is a BOM all processors must treat the entity as
UTF-16-encoded.
Omitting the charset parameter is not recommended for application/...
when used with transports other than HTTP or HTTPS. text/... should
not be used for 16-bit MIME with transports other than HTTP or HTTPS
(see discussion above (Section 9.2, Paragraph 6)).
9.5. Omitted Charset, no Internal Encoding Declaration and UTF-8 Entity
Content-type charset: [none]
<?xml version='1.0'?>
In this example, the charset parameter has been omitted, there is no
internal encoding declaration, and there is no BOM. Since there is
no BOM or charset parameter, the XML processor follows the
requirements in section 4.3.3, and optionally applies the mechanism
described in Appendix F (which is non-normative) of [XML] to
determine the charset encoding of UTF-8. Although the XML MIME
entity does not contain an encoding declaration, the encoding
actually _is_ UTF-8, so this is still a conforming XML MIME entity.
An XML-unaware MIME processor should make no assumptions about the
charset of the XML MIME entity.
See Section 9.1 for transport-related issues for UTF-8 XML MIME
entities.
9.6. UTF-16BE Charset
Content-type charset: charset="utf-16be"
<?xml version='1.0' encoding='utf-16be'?>
Observe that the BOM does not exist. Since the charset parameter is
provided and there is no BOM, MIME and XML processors must treat the
enclosed entity as UTF-16BE encoded.
See also the additional considerations in the UTF-16 example
(Section 9.2) above.
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Content-type charset: charset="iso-2022-kr"
<?xml version="1.0" encoding="iso-2022-kr"?>
This example shows the use of a non-UTF charset (in this case Hangul,
but this example is intended to cover all non-UTF-family character
sets). Since the charset parameter is provided and there is no BOM,
all processors must treat the enclosed entity as encoded per RFC1557.
Since ISO-2022-KR [RFC1557] has been defined to use only 7 bits of
data, no content-transfer-encoding is necessary with any transport:
for character sets needing 8 or more bits, considerations such as
those discussed above (Section 9.1, Section 9.2) would apply.
9.8. Omitted Charset with Internal Encoding Declaration
Content-type charset: [none]
<?xml version='1.0' encoding="iso-10646-ucs-4"?>
In this example, the charset parameter has been omitted, and there is
no BOM. However, the XML MIME entity does have an encoding
declaration inside the XML MIME entity that specifies the entity's
charset. Following the requirements in section 4.3.3, and optionally
applying the mechanism described in Appendix F (non-normative) of
[XML], the XML processor determines the charset encoding of the XML
MIME entity (in this example, UCS-4).
An XML-unaware MIME processor should make no assumptions about the
charset of the XML MIME entity.
For character sets needing 8 or more bits, considerations such as
those discussed above (Section 9.1, Section 9.2) would apply
9.9. INCONSISTENT EXAMPLE: Conflicting Charset and Internal Encoding Declaration
Content-type charset: charset="iso-8859-1"
<?xml version="1.0" encoding="utf-8"?>
Although the charset parameter is provided in the Content-Type header
and there is no BOM and the charset parameter differs from the XML
encoding declaration, MIME and XML processors will interoperate.
Since the charset parameter is authoritative in the absence of a BOM,
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all processors will treat the enclosed entity as iso-8859-1 encoded.
That is, the "UTF-8" encoding declaration will be ignored.
Processors generating XML MIME entities must not label conflicting
charset information between the MIME Content-Type and the XML
declaration unless they have definitive information about the actual
encoding, for example as a result of systematic transcoding. In
particular, the addition by servers of an explicit, site-wide charset
default has frequently lead to interoperability problems for XML
documents.
9.10. INCONSISTENT EXAMPLE: Conflicting Charset and BOM
Content-type charset: charset="iso-8859-1"
{BOM}<?xml version="1.0"?>
Although the charset parameter is provided in the Content-Type
header, there is a BOM, so MIME and XML processors may not
interoperate. Since the BOM parameter is authoritative for XML
processors, they will treat the enclosed entity as UTF-16-encoded.
That is, the "iso-8859-1" charset parameter will be ignored. XML-
unaware MIME processors on the other hand may be unaware of the BOM
and so treat the entity as encoded in iso-8859-1.
Processors generating XML MIME entities must not label conflicting
charset information between the MIME Content-Type and an entity-
initial BOM.
10. IANA Considerations
As described in Section 8, this specification updates the [RFC6839]
registration for XML-based MIME types (the "+xml" types).
11. Security Considerations
XML MIME entities contain information which may be parsed and further
processed by the recipient. These entities may contain, and
recipients may permit, explicit system level commands to be executed
while processing the data. To the extent that a recipient
application executes arbitrary command strings from within XML MIME
entities, they may be at risk.
In general, any information stored outside of the direct control of
the user -- including CSS style sheets, XSL transformations, XML-
entity declarations, and DTDs -- can be a source of insecurity, by
either obvious or subtle means. For example, a tiny "whiteout
attack" modification made to a "master" style sheet could make words
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in critical locations disappear in user documents, without directly
modifying the user document or the stylesheet it references. Thus,
the security of any XML document is vitally dependent on all of the
documents recursively referenced by that document.
The XML-entity lists and DTDs for XHTML 1.0 [XHTML], for instance,
are likely to be a commonly used set of information. Many developers
will use and trust them, few of whom will know much about the level
of security on the W3C's servers, or on any similarly trusted
repository.
The simplest attack involves adding declarations that break
validation. Adding extraneous declarations to a list of character
XML-entities can effectively "break the contract" used by documents.
A tiny change that produces a fatal error in a DTD could halt XML
processing on a large scale. Extraneous declarations are fairly
obvious, but more sophisticated tricks, like changing attributes from
being optional to required, can be difficult to track down. Perhaps
the most dangerous option available to attackers, when external DTD
subsets or external parameter entities or other externally-specified
defaulting is involved, is redefining default values for attributes:
e.g. if developers have relied on defaulted attributes for security,
a relatively small change might expose enormous quantities of
information.
Apart from the structural possibilities, another option, "XML-entity
spoofing," can be used to insert text into documents, vandalizing and
perhaps conveying an unintended message. Because XML permits
multiple XML-entity declarations, and the first declaration takes
precedence, it is possible to insert malicious content where an XML-
entity reference is used, such as by inserting the full text of
Winnie the Pooh in place of every occurrence of &mdash;.
Security considerations will vary by domain of use. For example, XML
medical records will have much more stringent privacy and security
considerations than XML library metadata. Similarly, use of XML as a
parameter marshalling syntax necessitates a case by case security
review.
XML may also have some of the same security concerns as plain text.
Like plain text, XML can contain escape sequences that, when
displayed, have the potential to change the display processor
environment in ways that adversely affect subsequent operations.
Possible effects include, but are not limited to, locking the
keyboard, changing display parameters so subsequent displayed text is
unreadable, or even changing display parameters to deliberately
obscure or distort subsequent displayed material so that its meaning
is lost or altered. Display processors SHOULD either filter such
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material from displayed text or else make sure to reset all important
settings after a given display operation is complete.
Some terminal devices have keys whose output, when pressed, can be
changed by sending the display processor a character sequence. If
this is possible the display of a text object containing such
character sequences could reprogram keys to perform some illicit or
dangerous action when the key is subsequently pressed by the user.
In some cases not only can keys be programmed, they can be triggered
remotely, making it possible for a text display operation to directly
perform some unwanted action. As such, the ability to program keys
SHOULD be blocked either by filtering or by disabling the ability to
program keys entirely.
Note that it is also possible to construct XML documents that make
use of what XML terms "[XML-]entity references" to construct repeated
expansions of text. Recursive expansions are prohibited by [XML] and
XML processors are required to detect them. However, even non-
recursive expansions may cause problems with the finite computing
resources of computers, if they are performed many times. For
example, consider the case where XML-entity A consists of 100 copies
of XML-entity B, which in turn consists of 100 copies of XML-entity
C, and so on.
12. References12.1. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2781] Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO
10646", RFC 2781, February 2000.
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